Broadband line drivers as ADSL (Asymmetric Digital Subscriber Line) line drivers, VDSL (Very High Speed Digital Subscriber Line) line drivers or others, generally called xDSL line drivers (where “x” represents the type of technology), are known to a person skilled in the art, e.g. from Bill Schweber, “Analog front ends bridge the xDSL-to-real-world chasm”, EDN Apr. 1, 1999 p. 48–64 (FIG. 3), from E. Nash, “Line-driver design for broadband communications applications”, Electronic Design, Dec. 1, 1997, p. 81–94, and from M. Steffes, “Optimizing performance in an xDSL line driver”, Electronic Design, Apr. 19, 1999, Vol. 47 No. 8, p. 44–58.
Such broadband line drivers typically comprise a cascade connection of an operational amplifier and a transformer with a rather low (e.g. 1:2) transformation ratio for galvanically isolating the amplifier from the line. The transformation ratio has to remain low, otherwise linearity and frequency response are limited because a high turns ratio in transformers is more prone to distortion and limits the bandwidth. Moreover, in many front ends, a high step up for the transmitted signal (=the signal going out on the line) means a high step down for the received signal, which affects the signal to noise ratio (SNR).
Line drivers are usually back-terminated, because signals traversing an unterminated cable are reflected and these reflections can severely affect the primary signal. Conventionally, the termination resistance cannot be implemented with sufficient precision if a high transformation ratio is used. Indeed, to correctly terminate the line, it is necessary to set the output impedance of the amplifier to be equal to the impedance of the line being driven. If the transformation ratio is rather high, e.g. 1:5, then the impedance reflected from the line is low, equal to the line characteristic impedance for the example given divided by 25. The low resistor values needed for the line impedance matching circuit in that case cannot be realised with sufficient resolution.
The immediate effect of back-termination is that the signal from the amplifier is halved before it is applied to the line. This doubles the power that the amplifier must deliver.
The maximum required line voltage depends on the modulation scheme used and on the line impedance and is for a low transformation ratio (1:1 or 1:2) generally between 15 and 30 V. Therefore, the operational amplifier has to generate a high voltage output signal, and thus a high power supply voltage (generally between 12 and 15 V) is required for the operational amplifier.
An active back-terminated broadband line driver is descried in EP-0 901 221. The active back-termination enables to substantially decrease the power consumption of the line driver, and is widely used in industry nowadays.
However, as explained before, high power supply voltages are still needed, and a consequence thereof is that integration of the line driver on chip is not possible or at least difficult to realise. The line driver therefore, at present, is not integrated in any of the chips.